The hand-written PDF writer uses the standard Courier fonts
(WinAnsiEncoding), so any character outside CP1252 - a Polish, Greek,
Cyrillic or CJK script - was silently written as '?'. Render now
collects the characters it had to replace and returns them, and the
export dialog lists them in a warning once the file is written.
Embedding a TrueType font (e.g. Courier Prime) with a CID/ToUnicode
map would fix the root cause; the warning keeps the zero-dependency
trade-off honest in the meantime.
Kharec